Message-id: <006b01cc9ade$48f53b30$dadfb190$%szyprowski@samsung.com>
Date:	Fri, 04 Nov 2011 11:41:18 +0100
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	'Mel Gorman' <mel@....ul.ie>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	'Michal Nazarewicz' <mina86@...a86.com>,
	'Kyungmin Park' <kyungmin.park@...sung.com>,
	'Russell King' <linux@....linux.org.uk>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	'KAMEZAWA Hiroyuki' <kamezawa.hiroyu@...fujitsu.com>,
	'Ankita Garg' <ankita@...ibm.com>,
	'Daniel Walker' <dwalker@...eaurora.org>,
	'Arnd Bergmann' <arnd@...db.de>,
	'Jesse Barker' <jesse.barker@...aro.org>,
	'Jonathan Corbet' <corbet@....net>,
	'Shariq Hasnain' <shariq.hasnain@...aro.org>,
	'Chunsang Jeong' <chunsang.jeong@...aro.org>,
	'Dave Hansen' <dave@...ux.vnet.ibm.com>
Subject: RE: [PATCH 6/9] drivers: add Contiguous Memory Allocator

Hello,

On Tuesday, October 18, 2011 3:43 PM Mel Gorman wrote:

> On Thu, Oct 06, 2011 at 03:54:46PM +0200, Marek Szyprowski wrote:
> > The Contiguous Memory Allocator is a set of helper functions for DMA
> > mapping framework that improves allocations of contiguous memory chunks.
> >
> > CMA grabs memory on system boot, marks it with CMA_MIGRATE_TYPE and
> > gives back to the system. Kernel is allowed to allocate movable pages
> > within CMA's managed memory so that it can be used for example for page
> > cache when DMA mapping do not use it. On dma_alloc_from_contiguous()
> > request such pages are migrated out of CMA area to free required
> > contiguous block and fulfill the request. This allows to allocate large
> > contiguous chunks of memory at any time assuming that there is enough
> > free memory available in the system.
> >
> > This code is heavily based on earlier works by Michal Nazarewicz.
> >
> > Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> > Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
> > CC: Michal Nazarewicz <mina86@...a86.com>
> > ---
> >  arch/Kconfig                         |    3 +
> >  drivers/base/Kconfig                 |   79 +++++++
> >  drivers/base/Makefile                |    1 +
> >  drivers/base/dma-contiguous.c        |  386 ++++++++++++++++++++++++++++++++++
> >  include/asm-generic/dma-contiguous.h |   27 +++
> >  include/linux/device.h               |    4 +
> >  include/linux/dma-contiguous.h       |  106 ++++++++++
> >  7 files changed, 606 insertions(+), 0 deletions(-)
> >  create mode 100644 drivers/base/dma-contiguous.c
> >  create mode 100644 include/asm-generic/dma-contiguous.h
> >  create mode 100644 include/linux/dma-contiguous.h
> >
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 4b0669c..a3b39a2 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -124,6 +124,9 @@ config HAVE_ARCH_TRACEHOOK
> >  config HAVE_DMA_ATTRS
> >  	bool
> >
> > +config HAVE_DMA_CONTIGUOUS
> > +	bool
> > +
> >  config USE_GENERIC_SMP_HELPERS
> >  	bool
> >
> > diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> > index 21cf46f..a5e6d75 100644
> > --- a/drivers/base/Kconfig
> > +++ b/drivers/base/Kconfig
> > @@ -174,4 +174,83 @@ config SYS_HYPERVISOR
> >
> >  source "drivers/base/regmap/Kconfig"
> >
> > +config CMA
> > +	bool "Contiguous Memory Allocator (EXPERIMENTAL)"
> > +	depends on HAVE_DMA_CONTIGUOUS && HAVE_MEMBLOCK && EXPERIMENTAL
> > +	select MIGRATION
> > +	select CMA_MIGRATE_TYPE
> > +	help
> > +	  This enables the Contiguous Memory Allocator which allows drivers
> > +	  to allocate big physically-contiguous blocks of memory for use with
> > +	  hardware components that do not support I/O map nor scatter-gather.
> > +
> > +	  For more information see <include/linux/dma-contiguous.h>.
> > +	  If unsure, say "n".
> > +
> > +if CMA
> > +
> > +config CMA_DEBUG
> > +	bool "CMA debug messages (DEVELOPEMENT)"
> 
> s/DEVELOPEMENT/DEVELOPMENT/
> 
> Should it be under DEBUG_KERNEL?
> 
> > +	help
> > +	  Turns on debug messages in CMA.  This produces KERN_DEBUG
> > +	  messages for every CMA call as well as various messages while
> > +	  processing calls such as dma_alloc_from_contiguous().
> > +	  This option does not affect warning and error messages.
> > +
> > +comment "Default contiguous memory area size:"
> > +
> > +config CMA_SIZE_ABSOLUTE
> > +	int "Absolute size (in MiB)"
> > +	depends on !CMA_SIZE_SEL_PERCENTAGE
> > +	default 16
> > +	help
> > +	  Defines the size (in MiB) of the default memory area for Contiguous
> > +	  Memory Allocator.
> > +
> > +config CMA_SIZE_PERCENTAGE
> > +	int "Percentage of total memory"
> > +	depends on !CMA_SIZE_SEL_ABSOLUTE
> > +	default 10
> > +	help
> > +	  Defines the size of the default memory area for Contiguous Memory
> > +	  Allocator as a percentage of the total memory in the system.
> > +
> 
> Why is this not a kernel parameter rather than a config option?

There is also a kernel parameter for CMA area size which overrides the value
from .config.
 
> Better yet, why do drivers not register how much CMA memory they are
> interested in and then the drive core figure out if it can allocate that
> much or not?

CMA area is reserved very early during boot process, even before the buddy 
allocator gets initialized. That time no device driver has been probed yet.
Such early reservation is required to be sure that enough contiguous memory
can be gathered and to perform some MMU related fixups that are required
on ARM to avoid page aliasing for dma_alloc_coherent() memory.

> > +choice
> > +	prompt "Selected region size"
> > +	default CMA_SIZE_SEL_ABSOLUTE
> > +
> > +config CMA_SIZE_SEL_ABSOLUTE
> > +	bool "Use absolute value only"
> > +
> > +config CMA_SIZE_SEL_PERCENTAGE
> > +	bool "Use percentage value only"
> > +
> > +config CMA_SIZE_SEL_MIN
> > +	bool "Use lower value (minimum)"
> > +
> > +config CMA_SIZE_SEL_MAX
> > +	bool "Use higher value (maximum)"
> > +
> > +endchoice
> > +
> > +config CMA_ALIGNMENT
> > +	int "Maximum PAGE_SIZE order of alignment for contiguous buffers"
> > +	range 4 9
> > +	default 8
> > +	help
> > +	  DMA mapping framework by default aligns all buffers to the smallest
> > +	  PAGE_SIZE order which is greater than or equal to the requested buffer
> > +	  size. This works well for buffers up to a few hundreds kilobytes, but
> > +	  for larger buffers it just a memory waste. With this parameter you can
> > +	  specify the maximum PAGE_SIZE order for contiguous buffers. Larger
> > +	  buffers will be aligned only to this specified order. The order is
> > +	  expressed as a power of two multiplied by the PAGE_SIZE.
> > +
> > +	  For example, if your system defaults to 4KiB pages, the order value
> > +	  of 8 means that the buffers will be aligned up to 1MiB only.
> > +
> > +	  If unsure, leave the default value "8".
> > +
> > +endif
> > +
> >  endmenu
> > diff --git a/drivers/base/Makefile b/drivers/base/Makefile
> > index 99a375a..794546f 100644
> > --- a/drivers/base/Makefile
> > +++ b/drivers/base/Makefile
> > @@ -5,6 +5,7 @@ obj-y			:= core.o sys.o bus.o dd.o syscore.o \
> >  			   cpu.o firmware.o init.o map.o devres.o \
> >  			   attribute_container.o transport_class.o
> >  obj-$(CONFIG_DEVTMPFS)	+= devtmpfs.o
> > +obj-$(CONFIG_CMA) += dma-contiguous.o
> >  obj-y			+= power/
> >  obj-$(CONFIG_HAS_DMA)	+= dma-mapping.o
> >  obj-$(CONFIG_HAVE_GENERIC_DMA_COHERENT) += dma-coherent.o
> > diff --git a/drivers/base/dma-contiguous.c b/drivers/base/dma-contiguous.c
> > new file mode 100644
> > index 0000000..e54bb76
> > --- /dev/null
> > +++ b/drivers/base/dma-contiguous.c
> > @@ -0,0 +1,386 @@
> > +/*
> > + * Contiguous Memory Allocator for DMA mapping framework
> > + * Copyright (c) 2010-2011 by Samsung Electronics.
> > + * Written by:
> > + *	Marek Szyprowski <m.szyprowski@...sung.com>
> > + *	Michal Nazarewicz <mina86@...a86.com>
> > + *
> > + * This program is free software; you can redistribute it and/or
> > + * modify it under the terms of the GNU General Public License as
> > + * published by the Free Software Foundation; either version 2 of the
> > + * License or (at your optional) any later version of the license.
> > + */
> > +
> > +#define pr_fmt(fmt) "cma: " fmt
> > +
> > +#ifdef CONFIG_CMA_DEBUG
> > +#ifndef DEBUG
> > +#  define DEBUG
> > +#endif
> > +#endif
> > +
> > +#include <asm/page.h>
> > +#include <asm/dma-contiguous.h>
> > +
> > +#include <linux/memblock.h>
> > +#include <linux/err.h>
> > +#include <linux/mm.h>
> > +#include <linux/mutex.h>
> > +#include <linux/page-isolation.h>
> > +#include <linux/slab.h>
> > +#include <linux/swap.h>
> > +#include <linux/mm_types.h>
> > +#include <linux/dma-contiguous.h>
> > +
> > +#ifndef SZ_1M
> > +#define SZ_1M (1 << 20)
> > +#endif
> > +
> > +#ifdef phys_to_pfn
> > +/* nothing to do */
> > +#elif defined __phys_to_pfn
> > +#  define phys_to_pfn __phys_to_pfn
> > +#elif defined __va
> > +#  define phys_to_pfn(x) page_to_pfn(virt_to_page(__va(x)))
> > +#else
> > +#  error phys_to_pfn implementation needed
> > +#endif
> > +
> 
> Parts of this are assuming that there is a linear mapping of virtual to
> physical memory. I think this is always the case but it looks like
> something that should be defined in asm-generic with an option for
> architectures to override.
> 
> > +struct cma {
> > +	unsigned long	base_pfn;
> > +	unsigned long	count;
> > +	unsigned long	*bitmap;
> > +};
> > +
> > +struct cma *dma_contiguous_default_area;
> > +
> > +#ifndef CONFIG_CMA_SIZE_ABSOLUTE
> > +#define CONFIG_CMA_SIZE_ABSOLUTE 0
> > +#endif
> > +
> > +#ifndef CONFIG_CMA_SIZE_PERCENTAGE
> > +#define CONFIG_CMA_SIZE_PERCENTAGE 0
> > +#endif
> > +
> > +static unsigned long size_abs = CONFIG_CMA_SIZE_ABSOLUTE * SZ_1M;
> 
> SIZE_ABSOLUTE is an odd name. It can't be a negative size. size_bytes
> maybe.
> 
> > +static unsigned long size_percent = CONFIG_CMA_SIZE_PERCENTAGE;
> > +static long size_cmdline = -1;
> > +
> > +static int __init early_cma(char *p)
> > +{
> > +	pr_debug("%s(%s)\n", __func__, p);
> > +	size_cmdline = memparse(p, &p);
> > +	return 0;
> > +}
> > +early_param("cma", early_cma);
> > +
> > +static unsigned long __init __cma_early_get_total_pages(void)
> > +{
> > +	struct memblock_region *reg;
> > +	unsigned long total_pages = 0;
> > +
> > +	/*
> > +	 * We cannot use memblock_phys_mem_size() here, because
> > +	 * memblock_analyze() has not been called yet.
> > +	 */
> > +	for_each_memblock(memory, reg)
> > +		total_pages += memblock_region_memory_end_pfn(reg) -
> > +			       memblock_region_memory_base_pfn(reg);
> > +	return total_pages;
> > +}
> > +
> 
> Is this being called too early yet? What prevents you seeing up the CMA
> regions after the page allocator is brought up for example? I understand
> that there is a need for the memory to be coherent so maybe that is the
> obstacle.

Right now we assume that CMA areas can be created only during early boot with 
memblock allocator. The code that converts memory on-fly into CMA region
can be added later (if required).
 
> > +/**
> > + * dma_contiguous_reserve() - reserve area for contiguous memory handling
> > + *
> > + * This funtion reserves memory from early allocator. It should be
> > + * called by arch specific code once the early allocator (memblock or bootmem)
> > + * has been activated and all other subsystems have already allocated/reserved
> > + * memory.
> > + */
> > +void __init dma_contiguous_reserve(phys_addr_t limit)
> > +{
> > +	unsigned long selected_size = 0;
> > +	unsigned long total_pages;
> > +
> > +	pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
> > +
> > +	total_pages = __cma_early_get_total_pages();
> > +	size_percent *= (total_pages << PAGE_SHIFT) / 100;
> > +
> > +	pr_debug("%s: total available: %ld MiB, size absolute: %ld MiB, size percentage: %ld
> MiB\n",
> > +		 __func__, (total_pages << PAGE_SHIFT) / SZ_1M,
> > +		size_abs / SZ_1M, size_percent / SZ_1M);
> > +
> > +#ifdef CONFIG_CMA_SIZE_SEL_ABSOLUTE
> > +	selected_size = size_abs;
> > +#elif defined(CONFIG_CMA_SIZE_SEL_PERCENTAGE)
> > +	selected_size = size_percent;
> > +#elif defined(CONFIG_CMA_SIZE_SEL_MIN)
> > +	selected_size = min(size_abs, size_percent);
> > +#elif defined(CONFIG_CMA_SIZE_SEL_MAX)
> > +	selected_size = max(size_abs, size_percent);
> > +#endif
> > +
> 
> It seems very strange to do this at Kconfig time instead of via kernel
> parameters.
> 
> > +	if (size_cmdline != -1)
> > +		selected_size = size_cmdline;
> > +
> > +	if (!selected_size)
> > +		return;
> > +
> > +	pr_debug("%s: reserving %ld MiB for global area\n", __func__,
> > +		 selected_size / SZ_1M);
> > +
> > +	dma_declare_contiguous(NULL, selected_size, 0, limit);
> > +};
> > +
> > +static DEFINE_MUTEX(cma_mutex);
> > +
> > +static void __cma_activate_area(unsigned long base_pfn, unsigned long count)
> > +{
> > +	unsigned long pfn = base_pfn;
> > +	unsigned i = count >> pageblock_order;
> > +	struct zone *zone;
> > +
> > +	VM_BUG_ON(!pfn_valid(pfn));
> 
> Again, VM_BUG_ON is an extreme reaction. WARN_ON_ONCE, return an error
> code and fail gracefully.
> 
> > +	zone = page_zone(pfn_to_page(pfn));
> > +
> > +	do {
> > +		unsigned j;
> > +		base_pfn = pfn;
> > +		for (j = pageblock_nr_pages; j; --j, pfn++) {
> 
> This is correct but does not look like any other PFN walker. There are
> plenty of examples of where we walk PFN ranges. There is no requirement
> to use the same pattern but it does make reviewing easier.
> 
> > +			VM_BUG_ON(!pfn_valid(pfn));
> > +			VM_BUG_ON(page_zone(pfn_to_page(pfn)) != zone);
> > +		}
> 
> In the field, this is a no-op as I would assume CONFIG_DEBUG_VM is not
> set. This should be checked unconditionally and fail gracefully if necessary.
> 
> > +		init_cma_reserved_pageblock(pfn_to_page(base_pfn));
> > +	} while (--i);
> > +}
> > +
> > +static struct cma *__cma_create_area(unsigned long base_pfn,
> > +				     unsigned long count)
> > +{
> > +	int bitmap_size = BITS_TO_LONGS(count) * sizeof(long);
> > +	struct cma *cma;
> > +
> > +	pr_debug("%s(base %08lx, count %lx)\n", __func__, base_pfn, count);
> > +
> > +	cma = kmalloc(sizeof *cma, GFP_KERNEL);
> > +	if (!cma)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	cma->base_pfn = base_pfn;
> > +	cma->count = count;
> > +	cma->bitmap = kzalloc(bitmap_size, GFP_KERNEL);
> > +
> > +	if (!cma->bitmap)
> > +		goto no_mem;
> > +
> > +	__cma_activate_area(base_pfn, count);
> > +
> > +	pr_debug("%s: returned %p\n", __func__, (void *)cma);
> > +	return cma;
> > +
> > +no_mem:
> > +	kfree(cma);
> > +	return ERR_PTR(-ENOMEM);
> > +}
> > +
> > +static struct cma_reserved {
> > +	phys_addr_t start;
> > +	unsigned long size;
> > +	struct device *dev;
> > +} cma_reserved[MAX_CMA_AREAS] __initdata;
> > +static unsigned cma_reserved_count __initdata;
> > +
> > +static int __init __cma_init_reserved_areas(void)
> > +{
> > +	struct cma_reserved *r = cma_reserved;
> > +	unsigned i = cma_reserved_count;
> > +
> > +	pr_debug("%s()\n", __func__);
> > +
> > +	for (; i; --i, ++r) {
> > +		struct cma *cma;
> > +		cma = __cma_create_area(phys_to_pfn(r->start),
> > +					r->size >> PAGE_SHIFT);
> > +		if (!IS_ERR(cma)) {
> > +			if (r->dev)
> > +				set_dev_cma_area(r->dev, cma);
> > +			else
> > +				dma_contiguous_default_area = cma;
> > +		}
> > +	}
> > +	return 0;
> > +}
> > +core_initcall(__cma_init_reserved_areas);
> > +
> > +/**
> > + * dma_declare_contiguous() - reserve area for contiguous memory handling
> > + *			      for particular device
> > + * @dev:   Pointer to device structure.
> > + * @size:  Size of the reserved memory.
> > + * @start: Start address of the reserved memory (optional, 0 for any).
> > + * @limit: End address of the reserved memory (optional, 0 for any).
> > + *
> > + * This funtion reserves memory for specified device. It should be
> > + * called by board specific code when early allocator (memblock or bootmem)
> > + * is still activate.
> > + */
> > +int __init dma_declare_contiguous(struct device *dev, unsigned long size,
> > +				  phys_addr_t base, phys_addr_t limit)
> > +{
> > +	struct cma_reserved *r = &cma_reserved[cma_reserved_count];
> > +	unsigned long alignment;
> > +
> > +	pr_debug("%s(size %lx, base %08lx, limit %08lx)\n", __func__,
> > +		 (unsigned long)size, (unsigned long)base,
> > +		 (unsigned long)limit);
> > +
> > +	/* Sanity checks */
> > +	if (cma_reserved_count == ARRAY_SIZE(cma_reserved))
> > +		return -ENOSPC;
> > +
> > +	if (!size)
> > +		return -EINVAL;
> > +
> > +	/* Sanitise input arguments */
> > +	alignment = PAGE_SIZE << max(MAX_ORDER, pageblock_order);
> > +	base = ALIGN(base, alignment);
> > +	size = ALIGN(size, alignment);
> > +	limit = ALIGN(limit, alignment);
> > +
> > +	/* Reserve memory */
> > +	if (base) {
> > +		if (memblock_is_region_reserved(base, size) ||
> > +		    memblock_reserve(base, size) < 0) {
> > +			base = -EBUSY;
> > +			goto err;
> > +		}
> > +	} else {
> > +		/*
> > +		 * Use __memblock_alloc_base() since
> > +		 * memblock_alloc_base() panic()s.
> > +		 */
> > +		phys_addr_t addr = __memblock_alloc_base(size, alignment, limit);
> > +		if (!addr) {
> > +			base = -ENOMEM;
> > +			goto err;
> > +		} else if (addr + size > ~(unsigned long)0) {
> > +			memblock_free(addr, size);
> > +			base = -EOVERFLOW;
> > +			goto err;
> > +		} else {
> > +			base = addr;
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * Each reserved area must be initialised later, when more kernel
> > +	 * subsystems (like slab allocator) are available.
> > +	 */
> > +	r->start = base;
> > +	r->size = size;
> > +	r->dev = dev;
> > +	cma_reserved_count++;
> > +	printk(KERN_INFO "CMA: reserved %ld MiB at %08lx\n", size / SZ_1M,
> > +	       (unsigned long)base);
> > +
> > +	/*
> > +	 * Architecture specific contiguous memory fixup.
> > +	 */
> > +	dma_contiguous_early_fixup(base, size);
> > +	return 0;
> > +err:
> > +	printk(KERN_ERR "CMA: failed to reserve %ld MiB\n", size / SZ_1M);
> > +	return base;
> > +}
> > +
> > +/**
> > + * dma_alloc_from_contiguous() - allocate pages from contiguous area
> > + * @dev:   Pointer to device for which the allocation is performed.
> > + * @count: Requested number of pages.
> > + * @align: Requested alignment of pages (in PAGE_SIZE order).
> > + *
> > + * This funtion allocates memory buffer for specified device. It uses
> > + * device specific contiguous memory area if available or the default
> > + * global one. Requires architecture specific get_dev_cma_area() helper
> > + * function.
> > + */
> > +struct page *dma_alloc_from_contiguous(struct device *dev, int count,
> > +				       unsigned int align)
> > +{
> > +	struct cma *cma = get_dev_cma_area(dev);
> > +	unsigned long pfn, pageno;
> > +	int ret;
> > +
> > +	if (!cma)
> > +		return NULL;
> > +
> > +	if (align > CONFIG_CMA_ALIGNMENT)
> > +		align = CONFIG_CMA_ALIGNMENT;
> > +
> > +	pr_debug("%s(cma %p, count %d, align %d)\n", __func__, (void *)cma,
> > +		 count, align);
> > +
> > +	if (!count)
> > +		return NULL;
> > +
> > +	mutex_lock(&cma_mutex);
> > +
> > +	pageno = bitmap_find_next_zero_area(cma->bitmap, cma->count, 0, count,
> > +					    (1 << align) - 1);
> > +	if (pageno >= cma->count) {
> > +		ret = -ENOMEM;
> > +		goto error;
> > +	}
> > +	bitmap_set(cma->bitmap, pageno, count);
> > +
> > +	pfn = cma->base_pfn + pageno;
> > +	ret = alloc_contig_range(pfn, pfn + count, 0, MIGRATE_CMA);
> > +	if (ret)
> > +		goto free;
> > +
> 
> If alloc_contig_range returns failure, the bitmap is still set. It will
> never be freed so now the area cannot be used for CMA allocations any
> more.

There is bitmap_clear() call just after free: label, so the bitmap is updated
correctly.

> > +	mutex_unlock(&cma_mutex);
> > +
> > +	pr_debug("%s(): returned %p\n", __func__, pfn_to_page(pfn));
> > +	return pfn_to_page(pfn);
> > +free:
> > +	bitmap_clear(cma->bitmap, pageno, count);
> > +error:
> > +	mutex_unlock(&cma_mutex);
> > +	return NULL;
> > +}
> > +
> > +/**
> > + * dma_release_from_contiguous() - release allocated pages
> > + * @dev:   Pointer to device for which the pages were allocated.
> > + * @pages: Allocated pages.
> > + * @count: Number of allocated pages.
> > + *
> > + * This funtion releases memory allocated by dma_alloc_from_contiguous().
> > + * It return 0 when provided pages doen't belongs to contiguous area and
> > + * 1 on success.
> > + */
> > +int dma_release_from_contiguous(struct device *dev, struct page *pages,
> > +				int count)
> > +{
> > +	struct cma *cma = get_dev_cma_area(dev);
> > +	unsigned long pfn;
> > +
> > +	if (!cma || !pages)
> > +		return 0;
> > +
> > +	pr_debug("%s(page %p)\n", __func__, (void *)pages);
> > +
> > +	pfn = page_to_pfn(pages);
> > +
> > +	if (pfn < cma->base_pfn || pfn >= cma->base_pfn + cma->count)
> > +		return 0;
> > +
> > +	mutex_lock(&cma_mutex);
> > +
> > +	bitmap_clear(cma->bitmap, pfn - cma->base_pfn, count);
> > +	free_contig_pages(pfn, count);
> > +
> > +	mutex_unlock(&cma_mutex);
> 
> It feels like the mutex could be a lot lighter here. If the bitmap is
> protected by a spinlock, it would only need to be held while the bitmap
> was being cleared. free the contig pages outside the spinlock and clear
> the bitmap afterwards.
> 
> It's not particularly important as the scalability of CMA is not
> something to be concerned with at this point.

This mutex also serializes cma allocations, so there is only one alloc_contig_range()
call processed at once. This is done to serialize isolation of page blocks that is
performed inside alloc_contig_range().

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center

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